In certain automotive applications, such as head lamp adjusters, linear actuators are used for the dynamic regulation of devices. Linear displacement is provided by a threaded connection between the rotor core of a stepper motor and the output shaft. The output shaft is prevented from rotating thus rotation of the rotor causes the output shaft to move axially. The overall behavior of the actuator is strongly governed by the effects of friction.
The stepper motor is designed for smooth operation to reduce vibration and audible noise which is generally considered as undesirable. The friction of the rotational to linear conversion, that is the screw connection between the rotor and the output shaft, thus dominates the performance of the linear actuator.
There are two opposing characteristics of linear actuators, reversibility and irreversibility. Each being desirable in different applications. Reversibility is the ability of the motor to be back driven, that is the ability to move the output shaft by applying an external force to the output shaft. This is desirable where, for example, it may be necessary to manually move the device driven by the actuator during a power failure or a motor failure. Irreversibility is the ability of the linear actuator to resist movement of the output shaft when an external force is applied to the. output shaft. This is desirable where, for example, the device being moved by the actuator is subjected to external forces in normal operation (e.g. headlamp adjustor), where the actuator is required to hold position while the electric power to the motor is removed or reduced, or where security is important (e.g. door lock mechanism). Some applications require a combination of reversibility and irreversibility (e.g. external mirror adjusters) which require a high holding force to withstand vibration and wind forces during normal operation and yet be able to be manually adjustable during power failure or motor failure. Reversibility is related to the friction of the screw mechanism. Low friction gives good reversibility and high efficiency. High friction gives good load holding ability or irreversibility but the high friction also reduces the efficiency of the linear actuator. Thus a compromise between efficiency and load holding must be made.
Friction itself is related to temperature, to the relative speed of the gear elements and to surface roughness. Temperature and speed may change regularly and surface roughness changes as a result of wear. If the thread has a good efficiency it will lead to low energy loss, and if the thread has a bad efficiency it will lead to strong irreversibility. A self-locking screw thread, the term for a screw-nut system exhibiting irreversibility, will have a low thread angle (small screw thread pitch) yielding a screw-nut transmission efficiency of typically well below 50% due to high friction, often 30% or less. Such screws will not rotate (back drive) upon application of an external axial force.
Holding force is the maximum external force which can be applied axially to the output shaft without back driving the motor. That is, for a linear actuator, the force applied to the linear output shaft which the actuator can withstand or hold without moving. As explained above, holding force in a linear actuator is generally derived from mechanical friction of the screw connection.
Friction has a strong dependency on temperature. It is possible to have reversibility and irreversibility on the same thread but at different temperatures. This means, friction should be high enough to provide a self holding force against axial displacement (auto blocking) and to avoid bouncing at stall. However, friction should be low enough to avoid cold start issues, including high grease viscosity at low temperatures and blocking at stall. Bouncing is the condition where the motor, when driven into a hard stop, such as an end of travel, the stepper motor instead of stopping continues to rotate but in the opposite direction as though it has bounced back from the stop. Blocking is the condition where the motor is driven into a hard stop and becomes jammed, unable to be moved in either direction. Bouncing and blocking problems can be a consequence of or at least promoted by low efficiency transmission systems where friction is not controlled well.